In the case of a fuel cell system capable of highly-efficient small-scale electric power generation, it is easy to construct a system for utilizing heat energy generated during the electric power generation. Therefore, the fuel cell system is being developed as a distributed electric power generating system capable of realizing a high energy use efficiency.
The fuel cell system includes a fuel cell as a main body of its electric power generating section. Examples of such fuel cell are a phosphoric-acid fuel cell, a fused carbonate fuel cell, an alkaline fuel cell and a polymer electrolyte fuel cell. Among these fuel cells, the polymer electrolyte fuel cell is capable of generating electric power at a comparatively low temperature of about 130 degrees C. to 150 degrees C., and has features that are a high output density and a long life. Therefore, the polymer electrolyte fuel cell is expected to be applied to, for example, a power source of an electric car which requires a high output characteristic and a short start time at the same time, and a cogeneration system for domestic use which requires long-term reliability.
At the time of the electric power generating operation by the polymer electrolyte fuel cell, the fuel gas containing hydrogen is supplied to an anode side whereas the oxidizing gas containing oxygen is supplied to a cathode side. In the anode of the polymer electrolyte fuel cell, the supplied hydrogen is converted into electrons and protons. The electrons generated in the anode pass through an external load connected to the fuel cell system and reach the cathode of the polymer electrolyte fuel cell. The protons generated in the anode pass through a polymer electrolyte membrane and reach the cathode. Meanwhile, in the cathode of the polymer electrolyte fuel cell, water is generated by using the electrons having passed through the external load and reached the cathode, the protons having passed through the polymer electrolyte membrane and reached the cathode, and oxygen having been supplied to the cathode side. Note that the fuel gas is supplied from, for example, a fuel gas supplying device which generates hydrogen from a methane gas by a steam-reforming reaction. Moreover, the oxidizing gas is supplied from, for example, an oxidizing gas supplying device which takes in the air from the atmosphere by a sirocco fan.
Incidentally, in the fuel cell system including the polymer electrolyte fuel cell, in order to secure the conductivity of protons from the anode side to the cathode side, the polymer electrolyte membrane need to be maintained in a wet state. Therefore, in this fuel cell system, the humidified fuel gas and the humidified oxidizing gas are supplied to the anode side and the cathode side, respectively. Moreover, in this fuel cell system, in order to adequately secure the energy conversion efficiency when converting the free energy change of the chemical reaction into the electric energy, for example, the polymer electrolyte fuel cell is operated under such an operating condition (hereinafter referred to as “low humidification operating condition”) that mutual relationships that are Tcell>Tda and Tcell>Tdc are satisfied where Tda denotes the dew point of the fuel gas, Tdc denotes the dew point of the oxidizing gas, and Tcell denotes the temperature of the polymer electrolyte fuel cell. With this, the fuel cell system stably achieves a predetermined electric power generating performance for a long period of time (see Patent Document 1 for example).
Meanwhile, regarding the electric power generating operation of the fuel cell system, since it is unnecessary to carry out the electric power generating operation in a case where both the electric energy and the heat energy generated by the fuel cell system are unnecessary, the fuel cell system adopts a start-stop type operating method for starting or stopping the electric power generating operation of the polymer electrolyte fuel cell depending on the situation. In the start-stop type operating method, in the case where both the electric energy and the heat energy are unnecessary, a control apparatus of the fuel cell system stops operations of the fuel gas supplying device and the oxidizing gas supplying device, and then cuts off an electrical connection between the polymer electrolyte fuel cell and the external load. Thus, the polymer electrolyte fuel cell becomes an open circuit state. Then, in order to prevent the polymer electrolyte membrane from drying, the control apparatus encloses the humidified inactive gas in the polymer electrolyte fuel cell. Alternatively, the control apparatus cuts off connections between the polymer electrolyte fuel cell and the fuel gas supplying device and between the polymer electrolyte fuel cell and the oxidizing gas supplying device, and hermetically closes a fuel gas passage and an oxidizing gas passage. With this, the fuel cell system prevents the polymer electrolyte membrane from drying for a long period of time (see Patent Documents 2, 3 and 4 for example).
Patent Document 1: Japanese Patent Application 4-502749
Patent Document 2: Japanese Laid-Open Patent Application Publication 6-251788
Patent Document 3: Japanese Laid-Open Patent Application Publication 2004-163037
Patent Document 4: Japanese Laid-Open Patent Application Publication